It is well known to implement complex communication networks solely from electronic routing circuitry and point-to-point optical links. Further, long distance telephone networks use relatively long, heavily multiplexed, high bandwidth, point-to-point links to provide an economical solution for communication networks due to the concurrent and non-concurrent time sharing use of transmission and exchange facilities.
Typically, the traffic handled by telephone networks has a long duration compared with the necessary set-up time for making the circuit connection. On the other hand, computer networks have a burst transmission nature. Therefore, computer interconnects are implemented to have instantaneous access to a total available bandwidth much larger than is used for digital telephone transmissions.
Because computers transmit short messages compared to the telephone circuit connection time, computer oriented communications networks provide a different type of functionality than is offered by telephone networks. Particularly, if both the bandwidth and distance of a local area network (LAN) link is increased, the duration for "launching" a message or packet occupies a small time period compared to that for propagation of this message from one end of a link to the other.
For example, a one thousand bit packet on a 1 Gbs link can be launched in 1 microsecond. Conversely, light requires 5 microseconds to propagate 1 km in a fiber. Thus, if a data source node waited for an acknowledgment of receipt of the data before launching additional data, a total elapsed time of 10 microseconds is required before additional data is launched. Therefore, the effective data rate is not 1 Gbs, but only 100 Mbs. Further, if more than one data source node is connected to that same link (for cost sharing purposes), the connection arbitration procedures for link use would occupy an additional time period, thus further reducing the effective use of the link capacity. Consequently, any LAN using link capacities higher than 1 Gbs and link distances longer than 1 km must adhere to store and forward procedures to avoid reducing the channel utilization efficiency due to the short launch time and relatively long propagation time.
Because area networks have many transmitter/receiver locations (nodes) rather than just two end points as :n a point-to-point long distance telephone link, a problem arises in the interconnections between the nodes. A variety of architectures are known for sharing transmission facilities and connecting several nodes to a network. These architectures include, for example, full mesh, partial mesh, bus, passive star, active star, passive ring, active ring, central matrix switch, etc. However, all of these architectures encounter various problems in their implementation.
One such architecture where all nodes are coupled to a central point is the central matrix switch, which is used as an active hub to interconnect all nodes. The matrix switch provides multiple path connectivity between switch inputs and outputs and hence offers a network capacity greater than the capacity of an individual link. When the switch inputs are buffered, the matrix switch offers a means for connecting switches to switches and thus for creating fiber optic wide area networks (WANs). The wide area networks have good distance-bandwidth scalability because the central matrix switch can operate in the classical store and forward mode through the use of dedicated point-to-point links. The critical role of the node as a failure point is ameliorated by using redundancy procedures. Using conventional techniques, the matrix switch appears to be the best conventional approach for creating high bandwidth "backbone" networks, i.e., networks spanning distances of more than one kilometer with bandwidths in excess of 1 GHz.
However, the central matrix switch based fiber optic network has several problems. First, twice as many fiber optic links as communication channels are necessary, i.e., one link to carry data from a source node to the central matrix switch and a second link to carry data from the central matrix switch to the receiving node. Second, the central matrix switch approach does not alleviate the cost of the transmission plant i.e., the amount of cabled fiber, connectors and associated installation cost.
Another architecture uses a passive optical star coupler to interconnect the nodes. By using a separate optical frequency for each node, similar advantages as in the central matrix switch architecture are obtained. If there are as many nodes as there are inputs to the matrix switch, then that number or concurrent messages can be transported. The optical star coupler network is also similar to the matrix switch architecture in that each fiber carries only a single signal from a node to the passive star and only uses a single signal at a time from the passive star to the receiving node. The optical star network, however, uses a reliable passive hub approach which requires less optical sources than in the matrix switch architecture. However since all signals are broadcast to all receiver locations, full connectivity requires a total number of receivers equal to the square of the number of nodes. Each receiver continually monitors the network at a particular frequency, collects messages destined for its host location and drops all other messages. A serious disadvantage of the optical star architecture is that if the number of nodes is large, the inventory of receivers required to support the network goes well beyond the present commercial capabilities of the marketplace. Therefore, there is needed an architecture to achieve an actual reduction in fiber cable costs through wavelength multiplexing, to reduce the number of transmitters and receivers to be equal to the number of nodes, to reduce the number of discrete wavelength transmitters required, and to have a reliable electrically passive hub.
The matrix switch and optical star architectures provides parallel capacity equal to the bandwidth times the number of nodes and an efficient use of the optical links because of localized contention arbitration. However, the matrix switch architecture requires twice as many optical transmitters and receivers for a full load network capacity, and has an electrically active hub that is a central failure point. The optical star architecture requires many more receivers than nodes and a large number of different wavelength optical transmitters.